The dentist prescribed 15 amoxicillin 500mg for tooth infection, 1 pill every 8 hours? is that enough? after how many doses i can start feeling better?

Five day course is enough and you should feel better after 2 days provided the bacteria are sensitive to Amoxil. If the infection seems not improved after 2 days, the antibiotic should be changed.

Yes, that is a normal course of treatment. Make sure to take all 15 tablets. You should start to feel better in 48-72 hours. Antibiotic will reduce severity of infection, but will not fully cure. Additional den tac treatment (root canal Rx or extraction) will probably be required. Discuss this with your Dentist.